Colorimetry, or the science of color, has evolved over several centuries of experimentation to develop robust models for specifying human color perception. A good summary of the foundations of color science is provided in U.S. Patent Publication US-2002-0159098-A1, entitled “Hyperspectral System For Capturing Graphical Images”, published on Oct. 31, 2002 and hereby incorporated by reference.
Conventional apparatus for capturing colored graphical images utilize a method based upon an industrial implementation of a central color science concept, the Trichromatic Generalization, which explains how colors mix and match. In the conventional scheme, a coordinate system characterized as a Device Dependent Color Space (DDC) utilizes linear mixtures of three arbitrary primary colors to match the color of individual pixels of the original.
The origin of the scientific Trichromatic Generalization has its basis in human physiology. The sensation of color is a complex interaction of the human nervous system with light, electromagnetic radiation found between the wavelengths of 300 nm and 830 nm. Ordering the psychological designations of color perception creates the visible spectrum, from short to long wavelengths, violet, blue, green, yellow, orange, and red. The color matching rules of the Trichromatic Generalization are used to predict how mixtures of the different wavelengths are perceived by humans. Complicating the mechanical aspects of color perception are visual system anomalies.
The human eye's lens brings different wavelengths of light to focus at different distances behind the lens and absorbs almost twice as much blue light as yellow or red, resulting in a relative insensitivity to shorter wavelengths, a condition exaggerated by age. The light that finally passes through the eye strikes the retina, a small area at the back of the eye densely packed with individual light sensitive receptors connected to the optic nerve, the conduit that transmits and processes visual sensations from the eye to the visual cortex in the brain. It has been shown the light sensitive photoreceptors are of two kinds, rods, which function at night or at very low light levels, and cones, which function under daylight conditions and are the sole source of color perception sensations in humans. The cones are circularly situated at the center of the eye's focal area, the fovea, with the rods forming a ring around the cones.
The notion of “tri” associated with the Trichromatic Generalization arises from the relative sensitivity of the three different cone types generally accepted to be found within the fovea. About 64% of cones exhibit peak sensitivity to 575 nm wavelength light and are said to be red sensitive, though the 575 nm bandpass is actually perceived as yellow. Thirty two percent of cones are considered green, most sensitive to 535 nm light, and only two percent are blue, having a peak response at about 445 nm. It is generally believed analyzing the ratio of the neural activities generated by visually stimulating the three different photoreceptors is the method by which the human visual system interprets color. In practice, it has been shown that the channels of information from the three cones are transformed into three new so-called opponent channels, transmitting a red to green ratio, a yellow to blue ratio and a brightness factor, based upon red and green only, to the brain's visual cortex. The physiological sensations produced by visual stimulus are thought to be correlated with stored psychological perceptions, creating color vision.
The above described physiology allows perception of the physical aspects of color, electromagnetic radiation found between the wavelengths of 380 nm and 780 nm, referred to here as human-visible light. Physically, color perception varies according to the wavelength of the visual stimulus. Wavelength is calibrated in nm (nanometer) denominated units, with groups or multiple wavelengths described as bandwidth. When the bandpass of the bandwidth is narrow, the resulting perceptions are associated with pure, or highly saturated, color. As the observed bandpass widens, the color appears less pure. Observers with normal color vision generally identify pure blue as light with a wavelength of about 470 nm, pure green as light with a wavelength of about 535 nm, pure yellow as 575 nm light, and pure red as 610 nm light. However, individual observers often respond differently to the same specimen, so what is a pure color to one may not be perceived that way by another observer.
Besides wavelength, other important physical attributes of visible light are luminance, illuminance, transmittance (reflectance) and metamerism. Luminance accounts for light emitted, such as from a computer display, calibrated in units that reflect the eye's uneven sensitivity to different wavelengths. Illuminance is a measurement of the amount of light that falls on an observed object and transmittance (reflectance) is the measurement of light photons that are absorbed and regenerated as new photons in proportion to the amount of original photons that transmitted through (reflected off) the surface of the object. Various wavelengths of light that are absorbed and retransmitted through (reflected off) a measured image (or specimen) and presented as a percentage of the wavelengths of light that initially struck it can be described as the image's (specimen's) characteristic spectral transmittance (reflectance) curve.
It is useful to consider that the reproduction of a colored image may be thought of as an exercise in color matching which takes into account the spectral power distribution of the light source (ie: viewing conditions) illuminating the original, the characteristic curve of the original, the power distribution of the light source illuminating the reproduction, and the characteristic curve of the reproduction. When the characteristic curve of the source's power distribution is combined with the spectral transmittance of the specimen, a visual stimulus is created which triggers color perception. Mathematically characterizing the color perception triggered by the combination of a source's power distribution and a specimen's transmittance curve is a necessary first step in successfully reproducing the perception.
There is, however, a phenomenon that impacts color perception and therefore color reproduction; metamerism. To illustrate the phenomenon, consider two specimens with identical characteristic curves. They will appear to the average observer to match under any source of illuminance. Now, consider two specimens with different curves. They will appear to vary with regards to one another as the source of the illumination is varied. However, there can be two specimens that appear to match despite having different characteristic curves. This is metamerism. An example of metamerism is when the two specimens with different characteristic curves are observed under different sources of illumination, and a match is observed under one of the sources. Because the reproduction of colored images entails taking into account different viewing conditions and media, the mathematical characterization of a color perception destined for reproduction must take into account metameric matches. A color measurement system capable of identifying and predicting metamerism is the CIE system (devised by the Commission Internationale de l'Éclairage).
The CIE system includes non-linear color model transformations and procedures to account for different viewing conditions and visual phenomena such as metamerism and color contrast. And, to simplify color matching, the CIE system uses mathematical means, imaginary primaries designated X, Y and Z, to eliminate color matching possibilities that require a minus primary value to make a match. The X, Y and Z primaries create a superset of color which includes all colors a human might perceive. This is a key difference as compared to the physical primaries integrated into current graphical imaging systems, whose color gamut (or range of producible colors) is a subset of human color perception.
The three primary colors X, Y and Z utilized by the device independent CIE color model are mathematical abstractions based upon statistical analysis of the response of different observers to color specimens compared in a highly standardized manner. For example, the CIE has defined a standard manner for observing a color match which requires observing a structure free specimen field that subtends 2° of arc when positioned 45 cm (18 inches) from the eye's iris. By correlating the results of these observations with precise and accurate measurements of a visual stimuli's physical color properties, a device independent system able to correctly measure human color perception is created.
Devices currently utilized to quantify color for reproduction means use color systems that require actual samples of real primary colors (usually red, green and blue, i.e. R, G, B) be present to make measurements. Light is transmitted through a colored object and through filters that isolate the primary colors. Upon exiting the primary filters the light, effected by the optical density and color of the object, as well as the three primary color filters, is measured and noted as three integer values, one each for the R, G and B primary component created by the device for the object measured. This method creates a measurement process tied to a specific physical color space, with all the inherent color gamut limitations of physical rather than imaginary primaries. The methods and techniques used to create and measure the R, G and B components of a physical color space vary from vendor to vendor and are without any common standards.
Although a convenient way to describe colors, the limitation of any device dependent system is that regardless of how the three primary colors are chosen, observer metamerism effects (where two objects appear to some observers or devices to have the same color, but to other observers or devices the same objects do not match) cannot be eliminated. Values expressed by a device dependent color system are accurate only within a truncated color space, and only if the exact same filters, lights, inks or pigments used to render a particular color are used as the physical primaries in the measuring device, which is an impossibility. That being the case, it has been recognized that more information than is contained in a device dependent color model is needed to produce accurate color reproduction.
Despite it's known inaccuracy, device dependent color-based measuring and rendering systems have been integrated into virtually all industrial and commercial applications related to the processes that are called upon to reproduce full color images, such as printing, photography and television. Over generations the conflict of accurately measuring and rendering with physical color systems has lead to extensive trade practices being established. These practices, commonly referred to as “color correction,” integrate human judgment with the physical color systems in a way that requires humans to make decisions to resolve or mask the inherent limitations of a physical color system. In physical color image scanning methods, humans are expected to compensate for differences between the color content of the original image, what a scanner can capture of the original color content, how the scanner describes what it captured, and how the captured data must be adjusted for use by various digital, xerographic and lithographic rendering processes.
By agreement, the CIE, (Commission Internationale de l'Eclairage), since 1913, has developed standards regarding how the Trichromatic Generalization is interpreted, as well as how color is measured and described. The underlying premise of the CIE system, referred to as CIE-31, is that the stimulus for color is provided by the proper combination of a source of light, an object, and an observer. In 1931 the CIE introduced standardization of the source and observer and the methodology to derive numbers that provide a measure of a color seen under a standard source of illumination by a standard observer. This standardization forms the foundation of modern colorimetry. CIE-31 uses a specimen's Characteristic Curve for the calculation of Tristimulus Values X, Y, and Z and Chromaticity Coordinates x and y. The CIE-76 recommendations establish transformations of the X, Y, and Z Tristimulus Values into nearly visually uniform color scales such as CIELAB, and also established a method to quantify differences between two color specimens.
CIELAB (L*a*b*), the result of a non-linear transformation of X, Y and Z, is an opponent-type system that assumes a color cannot be red and green at the same time, or yellow and blue at the same time, though it can be both red and yellow (ie: orange) or red and blue (ie: purple). Therefore, a specimen's redness or greenness can be expressed as a single number, called a*, which is positive if the color is red and negative if it is green. It follows that yellowness or blueness is designated by the coordinate b*, positive for yellow and negative for blue. The third coordinate, L*, is the lightness of the color.
The full benefit of the CIE system has not been taken advantage of by the graphic arts industry with regards to image scanning. Recently, devices capable of measuring 1 nm and 5 nm wide bandpasses of radiant energy (sometimes referred to as “hyperspectral” in the literature) have been developed (see, e.g. U.S. Patent Publication US-2002-0159098-A1). For example, the graphical image scanner disclosed in U.S. Patent Publication US-2002-0159098-A1 includes a light source to illuminate the graphical image, a collector to segment the image into a plurality of pixels and collect light emanating from the plurality of pixels, a hyperspectral analyzer to divide the collected light into a plurality of hyperspectral bandpasses and measure a light intensity for each of the hyperspectral bandpasses, a calculator to transform the measured light intensities for the plurality of hyperspectral bandpasses into a device-independent representation of color for each of the pixels, a processor with stored program control to format the device-independent color representations for the plurality of pixels as a digital data file, and a memory for storing the digital data file. This scanner does however incorporate complex electronic and electro-optical hardware which result in a scanner cost and footprint that exceeds requirements for may smaller enterprises. Accordingly, it would be desirable to develop a hyperspectral graphical image scanner of reduced size and cost suitable for smaller enterprises.